Undyne and the mystery of Sans
by MNLMFangirl
Summary: Even if we think that we know our friends rather well there is always something that we could end up learning about them. Undyne is about to learn this for herself as she comes across something about Sans that she's never known but could she be right about what it is or is she wrong about what she thinks that it is? Rated T for safety for at least one mentioned moment.
1. Chapter 1

Chapter one:

"Thanks for helping me out with this paperwork Sans." Undyne speaks up with full gratitude as the two work on their stacks of paper.

"It's no problem Undyne." Sans assures her as he moves the one he had just finished off to the side. "I mean I am usually in quite a bit of the underground so I think it was good that you asked me."

"Yeah I know. It's like you have a bunch of posts in the underground." Undyne confirms putting the paper she finished aside. "So you really were the best choice for this." She reads what's on the paper in front of her as she finishes speaking.

"Yeah I kinda do." Sans agrees before the two fall silent continuing to work. He somewhat enjoys paperwork because then he doesn't have to really do much.

The only sound heard in Undynes kitchen is the soft rustling of paper and the soft breaths of the two monsters. The place is otherwise so quiet that it almost seems like a lonely silence.

Undynes eyes skirt over the text on the paper in front of her but she swears she handed all in this subject to Sans. She shrugs it off as a slight mix up since it has nothing to do with what was on the last page. She wordlessly hands it or Sans holding it in front of his face on his right, it is the side she's sitting beside after all. She blinks in confused surprise as after a moment Sans had not grabbed the paper. She turns to him and slightly moves it closer to his face but he doesn't even react. Her work suddenly forgotten she decides to try something wondering what exactly she's witnessing. She moves the paper to go to an angle towards Sans's left eye and as soon as it is in front of his eye he reacts.

Sans blinks and very slightly jumps then takes the paper. "Ah. Is something wrong with this paper?" He asks taking a look at it then he blinks in realization. "Oh. I was wondering where this one ended up. It seemed like a part of the last page was missing."

"Yeah." Undyne responds not really processing what's around her as she replays what just happened in her mind over and over again. She tries to look at it in every angle she can but it all ends up to the same thing in the end. "It can't be... Could it?" She questions really quietly to herself not wanting Sans to hear her. She turns to her papers trying to get back to work but what she had seen is still bugging her. She decides that she has to look more into this new development about one of her dear friends.

The two of them do end up finishing the paper work but it is now kind of late when they finish.

"Do you just want to stay the night?" Undyne asks turning to look to Sans as they stand there. She has her hands on her hips while she is still clad in her usual relaxation clothes that she's been in all along.

"Nah. Thanks but Paps is probably waiting for me. He does this whenever I'm out late, even if I tell him." Sans waves it off. "Don't want to worry him too much."

"Okay." Undyne agrees. "Then I'll see you tomorrow. Just be careful."

"Will do." Sans responds in a goodbye tone then he heads off with that leaving Undyne standing there by the entrance to her home. He of course made sure that the door was closed behind himself and he heads off on his way to go home.

About halfway there Bonetrousle begins playing letting Sans know that it is his brother calling him. So he picks up while continuing to walk.

"Hey bro. What's up?" He asks causally not missing a single step.

"Brother are you coming home yet?" Papyrus questions with a hint of worry in his voice.

"Yeah I am. I'm walking there as we speak." Sans confirms closing his eyes for a moment with a smile before opening his eyes smile still in place.

"Oh great!" Papyrus's tone picks up into a much more happy one. "How far are you?"

"Won't be long now." Sans responds to the other. "I could always take a shortcut to be back faster."

"I would like that." Papyrus admits.

"Okay. I'll be back in a second." Sans confirms then they hang up and he teleports to their house.

The moment he appears in the living room Papyrus comes running over to him in which Sans greets him with a smile.

Sans is quickly scooped up into Papyrus's arms and he accepts it wrapping his legs around the others torso as best as possible and his arms around his neck.

It's not uncommon for them to be like this with Papyrus holding and carrying Sans. It's so common that no one bats an eye whenever they see the two do it. It's become a normal for them.

"Let's go to your room Papyrus." Sans suggests. "I think this calls for some fluffy bunny. Besides you need to get some sleep."

"Yes of course. It is time for a bed time story." Papyrus agrees and carries Sans upstairs heading right to his room. He places Sans down to sit on the bed. "Here. Since I need to change I'll get the book." He tells the other who nods softly.

"Sounds good Paps." Sans glances down as he slightly kicks his feet that are hanging over the edge. He knows that neither of them care about being naked in front of each other.

After all they've taken baths together, changed in front of each other and Papyrus even had Sans strip so he could wash his clothes. They're so comfortable that they can walk to their rooms not really wearing anything after a shower. They've seen each other's bodies enough that it's nothing new to them.

Sans glances up watching Papyrus put on the long sleeved shirt used for bed time.

It's a simple orange pyjama shirt with mahogany pyjama pants to go with it. That's Papyrus's pyjamas, he only has one pair of pyjamas.

With that all done Papyrus grabs the book then heads over to the bed handing it to Sans before climbing onto the bed.

"Okay Paps get comfortable." Sans tells Papyrus who gets under the covers settling in nicely. He opens the book once his brother has settled in. "Okay here we go Paps." He begins to read the story out loud making sure Papyrus can see the pictures as that's most of the story. "Fluffy bunny and his brother are playing a game. Where is Fluffy bunny? Fluffy bunny is hiding. Fluffy bunny's brother looks for him. Fluffy bunny can't be found. Can you see Fluffy bunny?"

"There he is by the tree!" Papyrus points at the page.

"Good job bro." Sans congratulates. "Oh look. Fluffy bunny is by the tree."

"Peek a boo!" Sans and Papyrus say in unison and Papyrus covers his eyes then moves his hands to reveal his eyes.

Sans continues reading. "Fluffy bunny hides again. Fluffy bunny's brother looks for Fluffy bunny. Where is Fluffy bunny? Can you see Fluffy bunny?"

"Oh! Oh! There!" Papyrus points again in excitement. "Behind that rock!"

"Yup." Sans confirms. "Oh look. Fluffy bunny is by the rock."

"Peek a boo!" The two speak together as Papyrus hides his eyes only to reveal them once more.

"Fluffy bunny hides again. Fluffy bunny's brother must find Fluffy bunny. Where is Fluffy bunny? Can you see Fluffy bunny?" Sans pauses to allow Papyrus to answer the books question.

"Under that bush!" Papyrus responds beaming in excitement.

"Mhm." Sans nods in confirmation. "Oh look. Fluffy bunny is by the bush."

"Peek a boo!" They match each other in time and Sans holds back a chuckle as Papyrus does the usual movement once again.

Sans turns the page to continue. "Fluffy bunny hides again. Fluffy bunny's brother looks for Fluffy bunny. Where is Fluffy bunny? Can you see Fluffy bunny?"

Papyrus nods and points at the image on the page almost ready to bounce in his bed. "In the flowers!"

Sans nods stars to bring the book up a little. "Oh look. Fluffy bunny is by the flowers."

"Peek a boo!" They repeat in unison as Sans does a similar motion that Papyrus does but he covers his face with the book then moves it down.

"Fluffy bunny hides one last time." Sans can see Papyrus's excitement is starting to peak and it makes him just smile more than he already is. "Fluffy bunny's brother must find Flufy bunny. Where is Fluffy bunny? Can you see Fluffy bunny?" He questions his brother who nods with enthusiasm for a second before pointing at the book.

"There! By the pond!"

"Oh look. Is that Fluffy bunny?" Sans smiles widely.

"Peek a boo!" The repeat is done one last time still in unison and they both do their little motions.

"It's Mama bunny. Mama bunny says it's time for food." Sans pauses a second to chuckle slightly as Papyrus slightly jumps before he continues. "Fluffy bunny and Fluffy bunny's brother now go inside to have food. Goodbye Fluffy bunny."

"Goodbye Fluffy bunny." Papyrus waves bye with a smile and Sans closes the book.

"The ending still got you huh?" Sans lightly teases.

"Such an ending always gets the great Papyrus! I have learned that it is unavoidable!"

"Yeah. Even if something does get you you're still the coolest brother ever." Sans uses his magic to put the book away. He makes sure that after he's done making sure he's comfortable he leans towards the other placing a kiss on his forehead. "Good night Papyrus."

"Good night Sans." Papyrus's eyes droop closed and as she slightly rolls onto his side he drifts off into the land of sleep.

Sans leaves the room quietly and shuts the door almost all the way only to head to his own room. He stuffs his hands into his pockets as he walks. "What a day." He slightly yawns to himself as he walks. He makes it to his room heading inside and shutting the door almost all the way behind himself. He leaves it open a little to try to listen for Papyrus. He's always done this since they were younger so if Papyrus had a nightmare there's a better chance of him hearing his brother. He crawls onto his bed and lays his head down going to sleep himself.


	2. Chapter 2

Chapter two:

The next day...

Papyrus is the first one up as usual. He heads downstairs still in his pyjamas to begin cooking. He is making pancakes when he pauses hearing a knock at the door. "Coming!" He flips the pancake on the pan then heads to the front door. He opens it wondering who it could be. "Ah Undyne. Good morning." He greets the female fish monster. "Come inside." He invites her in as he heads back towards the kitchen. "I'm making breakfast if you're hungry."

"Thanks Papyrus. I haven't eaten yet." Undyne responds moving into the house and shutting the door behind herself. She is wearing a black tank top, jeans and brown knee high boots that has the top folded in half. She heads to the kitchen finding Papyrus humming as he works to finish the pancakes. She leans against the counter one leg crossed over the other and her arms crossed over her chest. "I'll be joining you for your patrol this morning." She informs Papyrus who stops humming to listen. "I just would like to see your guys patrol and we'll do a little training as well."

"Of course Undyne." Papyrus agrees to that. "I think you will be pleased with it."

"I'm sure I will." Undyne can't help but smile softly.

It isn't long before Papyrus is finished cooking the pancakes. He puts an even stack on three plates so everyone gets the same amount. He grabs three forks and puts them on the plate edges on the right side; except the middle one which he puts on the left.

Undyne blinks when she sees this. "Um Papyrus. Why is the middle fork on the left?"

"Oh. It's easier on Sans to have it on the left." Papyrus explains it to her casually.

"Easier?" Undyne wonders if Papyrus might actually tell her what she's been suspecting.

Papyrus nods. "Yup. It's easier on him. He's usually still sleepy in the mornings and he always reaches out with his left hand. So I put his utensil on that side for him."

"Oh." Undyne slightly wonders if Papyrus knows more than he's letting on but surely if there really is something wrong then Papyrus would know... Right?

"I shall go fetch Sans. Feel free to dig in." Papyrus begins to head out of the kitchen. "Feel free to use anything in the fridge for toppings." He tells her with a slight wave then he is off to go wake Sans up.

Undyne heads to the fridge and finds some syrup there which she grabs and adds it to the far right plate. She leaves it on the counter for the other two and picks up her plate. She begins to dig in while she waits and she must admit that Papyrus can make some good pancakes even if they're burned on the outside. She is about halfway done when Papyrus and Sans enter the room.

Sans is only missing his sweater and he definitely looks rather sleepy. He does make it to the counter and picks up his fork.

"Usual amount?" Papyrus asks his brother picking up the syrup that's on Sans's right side.

"Yeah. Thanks bro." Sans confirms and Papyrus adds some syrup for him.

"Don't worry about it." Papyrus assures him and once done he adds some to his own before putting the syrup away.

The three dig in and eat their pancakes putting the dishes in a pile on the counter.

"These will be clean after work." Papyrus informs them. "Sans lets get dressed and get ready for our patrol! We will also be doing some training as well and I'll make sure that you join in on it. I mean it."

"Yeah sure. We haven't done training together in almost two weeks so I guess it is time to do it huh?" Sans agrees then he teleports away almost as if quickly disappearing.

"You had better be getting changed and not going to Geillbys." Papyrus says to himself then he heads off to get dressed himself.

Undyne settles on the couch planning out how she might test her theory while they are on the patrol. She just wants to know for sure.

After a little of waiting Papyrus and Sans come back over dressed how they usually are.

Undyne gets up off the couch. "Okay. First I want to do a little training. It's always good to keep our skills in top shape then it's patrol time." She lets Papyrus lead as she walks beside Sans as they head off to begin the patrol while finding a good spot for some training.

The snow crunching beneath their feet as they head away from Snowdin.

Once they are a good distance away Undyne sees the open area as a good place to train a little.

"Here is good." Undyne tells them and they come to a stop with the two turning to face the female fish monster. "It should be quick. I just want to see how your fighting skills are." She informs them with her arms crossed over her chest as she looks at the two that stand kind of in front of her. "I want to observe your technique. So we shall do a fake fight between you and Papyrus. We might keep it short as we have to do the patrol." She adds knowing that they have ground to cover waiting for them to walk through so she hopes to keep this from being the time of a full length fight.

"Just like our training sessions huh?" Sans says glancing to Papyrus who nods glancing back to him.

"Yes of course. Let's show Undyne how we do it! Nyeh heh heh!" Papyrus agrees so the brothers move away from Undyne to stand facing each other like a real fight.

Undyne crosses her arms over her chest as she watches them. "Whenever you two are ready. Don't mind me watching." She instructs and Papyrus gets ready.

Papyrus is standing with his right leg back and his arms out as if ready to summon an attack.

Sans on the other hand is standing there with his hands in pockets looking the same as ever.

Papyrus blinks letting both eyes glow orange with a wispy waving tail coming out from the sides facing away from his face and a black circle like a pupil. He throws a hand out then as he brings it back he does the same with the other.

Two bones with an orange glow fly at Sans spinning and Sans stands there.

Undyne blinks a little worried that Sans is going to take the hit. She however blinks in surprise as Sans suddenly vanishes. "Oh yeah. He can teleport." Her memory reminds her and she is not surprised.

Sans appears this time behind Papyrus and raises his left hand up into the air. His left eye is glowing blue with a black oval in the middle as well as a blue wispy waving tail like Papyrus.

Undyne makes a mental note within her mind about how Sans only lights up one eye unlike Papyrus who lights up both eyes.

Bones with a blue glow come up around the two skeleton brothers.

Sans sends the bones on either side towards Papyrus who jumps backwards avoiding the attack. He makes the bones that'll miss disappear then sends two spinning ones towards Papyrus.

Papyrus throws both arms up and two bones shoot up taking he hit.

The two bones stop spinning as they become impaled in the bones that Papyrus summoned.

Papyrus sends the bones that just took a hit straight for Sans.

Sans raises his hands and two bones appear from the ground with the bottom part still below the ground. He has them make an X that Papyrus's attack hits.

Papyrus's attack is sent back a little away from Sans and closer to Papyrus but getting stuck in the ground.

Sans lowers the defence he had put up back into the ground and the two look at each other obviously getting rather into this now. He puts his hand back into his pocket then he surrounds himself with blue magic lifting himself off of the ground.

Undyne honestly has never seen him do something like this before so it intrigues her as she wonders what he plans to do next.

Papyrus sends a bone attack with a zigzag patterned hole though Undyne doubted that Sans was fast enough to get through it.

Sans however moves himself forward with his magic managing to make it through the gap that was provided. He keeps going heading straight for Papyrus summing a bone into his left hand while Papyrus's attack that missed vanishes. He is getting close to his brother when Papyrus throws a hand out grabbing Sans's soul. He is brought down to the ground but manages to land on his feet soon being let go of though it had stopped his charging attack.

Papyrus however moves his eyes away from Sans making Undyne want to tell him to not turn away from an opponent.


	3. Chapter 3

Chapter three:

Undyne doesn't even get the chance to say anything as suddenly Papyrus is spinning around summoning a bone into his hand swinging it. She watches as a slightly small bone with a blue glow is knocked away making her realize that he had turned his attention to block the attack.

Sans leaps back and a bunch of bones come flying from above to swoop down quickly heading for the back and head area of Papyrus. He obviously didn't make enough that they would kill Papyrus if they all hit but it's an impressive sneak move that Undyne nods at.

Papyrus looks back and he braces himself, not having time to dodge, slightly bringing his arms up as if also getting ready to strike back after. He's hit by a few taking some damage while the rest fly past then vanish. He gives a determined look to Sans who has levitated himself off the ground again and shoots some bones at him causing them to come from both sides.

Undyne leans very slightly closer when she notices this. She wants to see how fast Sans reacts to the attack heading his way after all as it could give the data she was hoping for.

Sans waits a few moments then he slightly curls into a ball making bones appear around him blocking every single bone Papyrus sent.

Undyne mentally notes how Sans seems to be aware of attacks heading his way from either side so she isn't as sure as she once was that this might give her some clues. She starts to think about calling it off while she watches Sans straighten.

Sans raises his arm into the air once more and a bunch of bones appear behind himself like an incomplete wall.

Papyrus summons a larger slightly chipped here and there bone into his hand ready with a fire in his eyes probably out of excitement.

Sans throws his hand forward and sends a wave at Papyrus who zigzags to dodge the bones almost like the gap is a wave. He then snaps his fingers and four Gaster blasters appear with two on each side.

The Gaster blasters almost resemble the skeleton head of a dog or a dragon in shape. They are white with holes made like markings including a stripe from the forehead area down to the hole where the nose is. Their eyes are somewhat like Sans yes resemble a human's more with the black sclera, white outer ring or iris and a black pupil. They also have pointy looking teeth and mouths that don't really seem to be able to close completely.

Papyrus slightly glances from one side to the other almost as if looking around himself though he doesn't look any less confident than before.

The Gaster blasters open their mouths charging a beam before quickly firing it in which Papyrus jumps back to the centre area as the beams make a square then they stop. They seem to pull away then they come back to the middle firing again in which Papyrus jumps into the air landing on his feet on the one side of the beams that soon stop. They are about to take another formation when Undyne speaks up stepping forward.

"Alright I have seen enough. You both have good fighting skills." She compliments the brothers as she moves a little closer while they stop and turn to her.

The two relax letting any of their attacks disappear, their eyes stop glowing, as well as Sans putting his hand back into his sweater pocket.

"We want to save some of our stamina and magic in case a human comes by. So let's continue on our patrol." Undyne tells them giving them a smile as she speaks.

"Sure sounds good." Sans shrugs it off rather casually. "Then I can have time to take a nap."

"No naps Sans!" Papyrus warns a little sternly in which Sans glances to him.

"Aw come on bro don't get so worked up over it." Sans gives his brother that look that says that he has a pun coming up to bug his brother as that's what siblings do. "Napping is bonerific after all. It truly is relaxing."

"Sans I swear!" Papyrus slightly stomps his left foot on the ground while Sans laughs.

Undyne smiles softly to herself and she shakes her head slightly rolling her eyes. She isn't too fond of the puns like Papyrus but for some reason it is rather amusing to watch the two interact. She finds that they are always going to do something interesting or perhaps even funny at least once in a day so it is worth it to hang around them. She must admit though that some of the puns are a little funny even if she still finds them at least a little annoying.

"Let's go." Papyrus turns around knuckles of his curled into fists hands are on his hip bones that are covered by his bottoms. "And no more puns Sans." His eyes slightly narrow in a slightly accusing glare in which Sans chuckles softly for a moment.

"We'll see." Sans responds after his little chuckle and the group heads off to continue on their way to doing the patrol that the skeleton brothers do every morning.

As they walk Undyne glances to Papyrus seeing that he is concentrating on where he's going right now. She turns to Sans and since she's on his right side she decides to go for it. She slightly reaches over and waves her hand in front of Sans's right eye.

Sans doesn't even react so she moves her hand closer yet still nothing.

She moves her hand away finally sexing that she has enough evidence to say something. She also knows that others are less likely to hear them out here but on top of that she is getting a little impatient about knowing. "Say can I ask you a question?"

Papyrus looks over his shoulder to them and Sans turns to Undyne. "Sure." He responds in which the fish monster turns to him. "What do you want to ask?"

"It's about Sans." Undyne glances back to the shorter skeleton meeting his eyes. "Recently I've noticed something and I wasn't sure if I should bring it up but... Sans are you blind in your right eye?"

All of them have stopped walking at this point and Papyrus is facing the other two.


	4. Chapter 4

Chapter four:

"Yes. That is true." Papyrus speaks up in which Undyne and Sans glance to him. He looks a little nervous slightly glancing away while his hands twist together almost as if making circles around each other. He may look nervous but Undyne can tell from the look in his eyes that he's telling the truth right now as she's been around the two long enough to pick up on things. "We didn't really mean to keep it from you. It's just that… We or well I didn't know when a good time to say it was so I didn't say anything. I ended up leaving it be to just let Sans decide if he was going to say anything. I hope it's not a problem Undyne. But I'll have you know that it'll be okay. Sans knows what he's doing." He adds obviously a little worried that things will change now that this information has become known. He obviously doesn't want Sans to be taken from his current job just because he is blind on one side of his body.

"It didn't seem like an important thing to bring up." Sans adds with a shrug and as he speaks the other two turn to him. He probably felt like adding that to add his own part as to why it was never brought to the female fish monster's attention.

"Oh. It's okay. It's not a big deal or a problem. I was just curious." Undyne slightly waves it off raising her hands to her chest height to do so, Papyrus now relaxed upon hearing this, before lowering her hands back to her sides. "I never even noticed before so knowing now doesn't make me feel any different about him. It seems like he's dealt with it all his life." She adds a little bit extra after she had ensured her friends that it's not a problem and that nothing had changed by her knowing this new information about Sans. She feels like a part of the mystery of Sans has been solved though she feels like she hasn't gotten everything just yet as there is one thing she is missing.

"Most of my life." Sans corrects her casually as if it doesn't really matter but if she is to know then at least she should know the truth instead of just making assumptions. "I wasn't always half blind. My right eye did work many years ago believe it or not."

"Is it okay if I ask what happened?" Undyne questions rather curious as she even gives a slight tilt of her head for a moment in a rather questioning way. She just hopes that she isn't prying too much into this as she doesn't want to push her friends but she does want to know. She kinda hopes that they do answer because of her need to know what happened to make Sans go blind in one of his eyes.

"It's not something we like to go into detail about." Sans admits slightly glancing down almost making Undyne wonder if she pried too deep into the matter, more than what Sans is comfortable with. "But what happened was that our father, after our mother died when we were young, was more interested in his work as a scientist. I guess he blamed us for her death or something but he began using us as test subjects. He ended up blinding me when he shot a laser in my eye from a big machine. It caused my right eye to be broken so it made me blind on that side and it doesn't light up like my left one does. We're just glad he messed up during an experiment that we used as a chance to escape. We were the only ones that made it out." He tells her a summed up version of the story of what happened all those years ago to the two of them and how he had ended up becoming blind.

"Oh." Undynes ear fins drop in sadness as she can only imagine what pain and torture the two had to go through. "I'm sorry that happened and I'm sorry I asked bringing up bad memories." She apologizes sincerely truly feeling bad about prying into the matter. She wonders what she could do in order to make it up to her friends for causing them to remember such painful memories from the past that she bets they buried within themselves from it being too painful to bear.

"It's okay." Sans looks back to her not really meeting her eyes but looking at her but Undyne is able to see that there isn't as much pain in his gaze as she had expected there to be. "It doesn't hurt us as much as it did all those years ago. We survived on our own together and we've left it in the past. We don't rattle our bones over it anymore."

"Sans! But pun aside That's right." Papyrus nods in agreement, after giving a verbal warning and slight glare for a moment to his brother, as Undynes eyes flicker back over to him as he stands tall scarf waving as if in a wind behind him. He looks as confident as ever as he even brings a hand to his chest. "What's important is that we made it and that we're okay."

"Yes. Quite so." Undyne nods in agreement feeling a rush of happiness as her ear fins even peek up, that the two can leave the bad in past. Her thoughts about making it up to them run off where she can no longer see it as the two have left the matter behind themselves so it's not a big deal. She figures that she should just leave it in the past as well like how they are doing so. "Well." She slightly clears her throat slightly bringing a hand up to be by her mouth as she does so before lowering it. "Shall we continue with our patrol?" She suggests to her two skeleton friends.

Papyrus places a fist on his hip while his other hand has one finger pointing up and is at chest level as if he just remembered something. "Why yes of course! Onward with our patrol!" He turns around so he's facing away from the two, his hands falling to hang at his sides then he takes the lead once more as they head off on their way.

Undyne and Sans follow as they patrol but begin to head towards the sentry stations of the two skeletons where they will in the end part ways for the time being.

The end.

AN:

Mostly written on my phone and I stayed up late to finish it so hopefully I caught any autocorrects.

I hope you enjoyed! Reviews are awesome and flamers will be used to make cake!

Kitkat.


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